At the end of trading in London on Thursday shares in Glencore plc (LON:GLEN) was priced at 179p.

That affords the Swiss commodities trading and mining company a market worth of £25 billion or just under $39 billion.

That's down a full two-thirds in value since the company's flotation at the end of May 2011.

Just this year the counter has dropped 40% as its acquisition of coal and copper dependent Xstrata drags on the trading arm, which during previous downturns were still able to make money.

The tie-up was supposed to create a new breed of resource company – a vertically integrated mine-to-market enterprise able to take on the world's giant diversified miners.

Glencore's stock performance is also worse than that of BHP Billiton and Rio Tinto despite the Anglo-Australian miners reliance on iron ore (the steelmaking ingredient hardly features at Glencore) which together with crude is the worst performing commodity over the last two years.

But for mining investors there may be bigger worry than a depressed share price.

A new research note from JP Morgan Cazenove quoted in The Guardian warns that Glencore's debt – estimated to come in at a stomach churning $48 billion when the company reports half-year numbers next week – as the biggest issue facing the company.

"Bold borrowings aren’t quite what they seem, it should be said, because Glencore’s marketing division holds a stockpile of commodities as inventories that can be turned into cash. Viewed that way, net debt might be nearer $30.5bn at year-end, estimates JP Morgan Cazenove.

"But here’s the rub: Glencore might have to go ahead and turn some of that stock into cash if its wants to save its BBB credit rating. “At spot commodity prices, we calculate net debt needs to fall $16bn by year-end 2016 to safeguard Glencore’s BBB credit rating,” says JP Morgan.

"Preservation of BBB is a financial priority, Glencore said in March, for the sound reason that a healthy rating is vital to keep funding costs low in the trading-cum-marketing division."

Glencore has other ways to find some of that $16 billion in cash like nixing its dividend. But any large seller into an already depressed metals and minerals market can only make things worse.

• [Editor:Juan]